The present invention relates to apparatus and methods for sorting gemstones according to predetermined characteristics and, in particular, to apparatus and methods for testing and sorting, i.e., classifying, previously irradiated topaz gemstones according to their residual radiation levels.
The technique of irradiating white topaz in order to change its color to blue is described, e.g, in U.S. Pat. No. 4,749,869, issued to Fournier, which patent is incorporated herein by reference. Once topaz is irradiated it continues to be radioactive for some months afterwards. Before the irradiated topaz can be used by jewelers and others, i.e., before it can be placed in the public domain, its residual gamma radiation must be less than certain prescribed limits that are set by the Nuclear Regulatory Commission (NRC) or other regulatory agencies. These limits are very low, almost at the levels of common background radiation, making measurements of the residual radiation very difficult.
Hence, immediately after being irradiated (when the residual gamma radiation is the highest), the blue topaz must be stored in shielded containers until such time as the residual gamma radiation of each stone is within the prescribed limits. Because the residual gamma radiation varies as a function of stone size, shape and weight, the white topaz is usually presorted into batches of prescribed sizes, shapes, and weights (e.g., 3 mm, 4 mm, 5 mm, 6.times.4 mm, etc.) commonly used by jewelers and others in the industry prior to being irradiated, and remains sorted into such batches during and after irradiation.
After being held in shielded storage for an estimated time period (smaller-sized gemstones tend to retain their residual radiation longer than larger-sized gemstones), which may be four to twelve months, a given batch of blue topaz is retrieved from storage, and each gemstone within the batch is checked to see if its residual radiation is within the prescribed limits. If so, the gemstone is placed in a batch suitable for distribution. If not, the gemstone is placed in a batch that is returned to storage.
Because the prescribed limits on residual radiation are extremely low, it is necessary (when checking the radiation levels of each gemstone) to place the gemstone in a shielded container, e.g., a lead enclosure, where the residual radiation can be accurately measured in the absence of common background radiation. As indicated above, if the residual radiation is below the prescribed limits set by the NRC or other regulatory agency, the topaz is classified as acceptable. If the residual radiation is greater than the prescribed limits, the topaz is classified as not acceptable, and it is placed back in a shielded container and stored for an additional length of time, after which time its residual radiation is again measured.
Unfortunately, heretofore this process of checking the residual radiation of each blue topaz gemstone is extremely labor and time intensive. Not only must each stone or small parcel of stones be handled individually, in order to place the stone in a suitable shielded chamber where a radiation measurement may be made, but the radiation measurement itself takes several seconds, typically up to 20-30 seconds, to complete. Further, if statistical data associated with the radiation measurement is to be made, the logging and processing of such data may further slow down the measurement process. Hence, the rate at which the stones can be classified is severely limited. What is needed, therefore, is an apparatus and method that carries out this classifying process automatically, with minimal supervision, and at a high rate of throughput.